To alert the user that they can and should choose the right mode.
Speeds up the dumpmasterkey test *a lot*: Before: ok github.com/rfjakob/gocryptfs/gocryptfs-xray/xray_tests 0.398s After: ok github.com/rfjakob/gocryptfs/gocryptfs-xray/xray_tests 0.023s
The single test compares the gocryptfs-xray output with the expected output.